The preparation of ethylene copolymers in a solution polymerization process is well known. The use of a continuously stirred tank reactor (CSTR) in such solution processes is generally preferred because the CSTR provides good mixing of the solvent, monomer(s) and catalyst.
The polymerization of ethylene is highly exothermic. Accordingly, it is common practice to use ambient or chilled reactor feeds to maximize reactor productivity for a given reaction temperature.
The use of a CSTR with chilled feeds works especially well for ethylene homopolymerizations. However, CSTR's are not without limitations, especially for the preparation of thermoplastic ethylene copolymers having a density of from about 0.900 to 0.930 g/cc. One difficulty with the preparation of ethylene copolymers is that the reactivity of the ethylene is generally higher than that of the other alpha olefins (such as butene, hexene and octene) that are commonly used to prepare the copolymers. Thus, the ethylene is preferentially consumed; leaving a reactor effluent that is comonomer rich and typically still contains some active catalyst. This problem can be mitigated by adding another CSTR to receive the discharge from the first CSTR—and indeed, the use of two CSTR's is well known. Moreover, the use of two CSTR's also provides flexibility to adjust the molecular weight and/or comonomer distribution of the resulting polyethylene. However, even the discharge from the second CSTR used to prepare copolymers is generally also comonomer rich and often still contains active catalyst. The recovery of the residual comonomer from the polymer/solvent solution is understandably energy intensive. Accordingly, a cost effective method to recover the remaining comonomer and also, lower residual catalyst in the finished product would provide a useful addition to the art.